KABUL, Afghanistan — In a brazen attack in Kabul’s most secure district, a Taliban suicide bomber on a mission to target foreign-run medical teams infiltrated the capital’s main military hospital and killed at least six Afghan medical students and wounded 23 others.

The bombing was a blow to Afghan and NATO forces that have sharply expanded checkpoints and security cordons in the Afghan capital as the Taliban intensifies their attacks ahead of a planned U.S. drawdown in July. It also indicated the insurgency may now be favoring terror-style tactics rather than larger traditional battles.

The attack came as a related group — the Taliban in neighboring Pakistan — claimed responsibility for two attacks on tankers carrying fuel for NATO forces in Afghanistan. One assault was a twin strike: the first blast damaging a tanker and then a second explosion that killed 15 people trying to siphon fuel from the disabled vehicle in northwestern Pakistan. Later, 14 NATO tankers were gutted in a bombing at a nearby border town, but no one was hurt.

The Pakistani Taliban is a network of militant groups that is distinct from the Afghan Taliban, but linked by ideology. They help shelter militants who conduct cross-border attacks into Afghanistan.

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In the Kabul attack, no foreign medical doctors or nurses were among the dead or wounded, Afghan and NATO officials said. There are a number of military doctors and nurses from various NATO countries at the hospital as part of the alliance’s mission to train Afghan forces.

All those killed were eating lunch inside a tent used by medical students for meals, Defense Ministry spokesman Gen. Mohammed Zaher said.

The bombing was condemned by President Hamid Karzai and NATO. The United Nations called it a violation of “international humanitarian law.”

The blast came as the Taliban stepped up their attacks as part of their spring offensive against NATO, Afghan government installations and officials. Insurgents also have promised revenge attacks after the U.S. killing of al-Qaida leader Osama bin Laden in Pakistan earlier this month.

The effectiveness of the Taliban’s campaign will in part determine the size of U.S. President Barack Obama’s planned drawdown of American troops. He has said its size will depend on conditions on the ground.

The alliance has committed itself to handing over control of security to Afghans by 2014.

Taliban spokesman Zabiullah Mujahid said the attack targeted foreign trainers and Afghan doctors who work with them. He claimed two bombers took part, but Azimi said only one was involved at the Mohammad Daud Khan military hospital.

The Taliban have promised to carry out attacks in major population centers, and earlier this month tried to take over key government installations in the southern city of Kandahar — which was once their capital and stronghold. But that attack failed and more than two dozen militants were killed.

U.S. and NATO military officials, however, have questioned the Taliban’s ability to mount large operations. Thousands of insurgents, including midlevel commanders, have been killed or captured and hundreds of weapons caches seized during battles over the winter.

Lt. Gen. David Rodriguez, the second-ranking U.S. general in Afghanistan, predicted recently that the Taliban — having lost ground during the fall and winter — will employ more indirect tactics such as suicide attacks and assassinations.

Mujahid said that was part of the Taliban strategy against the government. “The mujahedeen are able to infiltrate into the ranks of the enemy and using opportunities are able to attack,” he said.

It was unclear if the bomber was a member of the hospital staff, but the ability of the attacker to get inside the heavily guarded hospital raised fresh concerns about possible infiltration of Afghan security forces. The facility is in one of Kabul’s most heavily protected neighborhoods and close to NATO headquarters, the U.S. Embassy and other diplomatic facilities. Security checks are stringent and all visitors are searched.

In the most embarrassing breach of a government facility, a Taliban militant opened fire inside the Afghan Defense Ministry on April 18, killing two Afghan soldiers. At the time, the Taliban said one of their agents who was also an army officer planned the attack to coincide with a visit of the French defense minister, who was not in the ministry at the time.

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Associated Press writers Jon Gambrell and Rahim Faiez in Kabul and Riaz Khan in Peshawar, Pakistan, contributed to this report.